Problem: A circle with area $81\pi$ has a sector with a $180^\circ$ central angle. What is the area of the sector? ${81\pi}$ $\color{#9D38BD}{180^\circ}$ ${\dfrac{81}{2}\pi}$
Answer: The ratio between the sector's central angle $\theta$ and $360^\circ$ is equal to the ratio between the sector's area, $A_s$ , and the whole circle's area, $A_c$ $\dfrac{\theta}{360^\circ} = \dfrac{A_s}{A_c}$ $\dfrac{180^\circ}{360^\circ} = \dfrac{A_s}{81\pi}$ $\dfrac{1}{2} = \dfrac{A_s}{81\pi}$ $\dfrac{1}{2} \times 81\pi = A_s$ $\dfrac{81}{2}\pi = A_s$